For a movie about a group of drug smugglers pretending to be an all-American clan, “We’re the Millers” hogs the middle of the road like the big, farty RV at its center. I guess this is what counts as a “family comedy” these days: R-rated, generically vulgar, not so much shocking as shockingly safe, despite the occasional swollen testicle and passing references to anal sex.

It’s an August dog-day special, in other words: a few easy laughs, one or two flashes of inspiration, and enough sentimentality to ensure that no one actually gets hurt. Jason Sudeikis plays David Clark, a Denver-based slacker who’s still selling pot in his mid-30s; when his apartment is robbed of all his profits, he’s forced by his slick businessman boss (Ed Helms) to drive to Mexico and bring back “a smidgen and a half” of prime weed.

The only way David figures he’ll get across the border unmolested is by posing as the head of a squeaky clean vacationing family. Promising a cut of the take, he hires a “wife” in out-of-work stripper Rose (Jennifer Aniston), a “son” in nerd-next-door Kenny (Will Poulter), and a “daughter” in foul-mouthed teen runaway Casey (Emma Roberts).

The casting alone tells you how edge-free “We’re the Millers” is. Aniston as a stripper who never takes her clothes off? (“You won’t have to get naked,” David tells Rose, and don’t worry — she doesn’t.) The eminently sensible Roberts as a street punk? Sudeikis tries to make a break from “Saturday Night Live” with his first leading role, but his funniest bits are still reactive — he’s no one’s idea of a leading man, let alone a drug dealer.

Some of us remember Poulter as the show-stealer from the 2007 ingenious British comedy “Son of Rambow,” and he’s the closest “We’re the Millers” comes to a wild card, even if the movie never lets him truly cut loose. The exception is when gangly Kenny raps along to the TLC chestnut “Waterfalls” with blissful immersion — but you already know that bit from the film’s trailer.

In fact, the trailer represents a tighter, funnier, better version of the feature film, even if it gives you less of Nick Offerman and Kathryn Hahn, wickedly amusing as another RV couple who are both straighter and kinkier than the Millers. Director Rawson Marshall Thurber (“Dodgeball,” “The Mysteries of Pittsburgh”) keeps the tone lightly raucous and utterly predictable, and “We’re the Millers” plays mostly like a defanged “Little Miss Sunshine,” to the point of imitating some of the earlier film’s camera setups.

The only time Thurber’s movie gets near actual comic subversion is when “mother” Rose and “daughter” Casey decide to teach “son” Kenny a few things about kissing. For a few queasy, hilarious seconds, it seems like “We’re the Millers” is about to turn into a vintage R. Crumb cartoon. And then it goes back to being just a cartoon.